Known methods of color proofing (prepress proofing) which use a photopolymer include an overlay method and a surprint method. In the overlay method, plural sheets having separation images of different color formed on transparent supports are prepared and then placed upon one another to conduct color proofing. In the surprint method, a multicolor image is obtained by successively forming separation images of different color on a single support.
Image-forming materials for use in both the above-mentioned overlay and surprint methods are described, for example, in JP-A-59-97140 (The term "JP-A" as used herein means an "unexamined published Japanese patent application"). JP-A-59-97140 discloses a photo-sensitive transfer sheet having a peeling layer made of an organic polymer and a photosensitive resin layer successively formed on a support (hereinafter occasionally referred to as a "temporary support"). A color proofing sheet having a multicolor image is obtained by exposing and developing the respective photosensitive transfer sheets to form separation images followed by repeatedly transferring the separation images to an arbitrary support.
In order to obtain good transfer properties, the characteristics of the peeling layer play an important role in the photo-sensitive transfer sheet comprising a support having thereon a peeling layer made of an organic polymer and a photosensitive resin layer in this order. Alcohol-soluble polyamides such as copolymerized nylon and N-alkoxymethyl nylon are known materials for such peeling layers. However, when an alcohol-soluble polyamide is used alone, adhesion of the peeling layer to the temporary support increases under humid conditions, because the peeling layer readily absorbs moisture. This occasionally causes a transfer failure. Furthermore, the peeling layer made of an alcohol-soluble polyamide tends to stains due to dyes, a pigment, etc. upon development, to thereby result in the so-called colorant fog.
To solve the above-described problems, JP-A-61-188537 proposes a photosensitive transfer sheet having a peeling layer comprising a mixture of an alcohol-soluble polyamide and a hydroxystyrene type polymer. According to this method, a peeling layer having excellent resistance to moisture can be formed on a support. As a result, even when the transfer step of a separation image is carried out in a high humidity environment, and after the transfer sheet having the image formed thereon is adhered to a support of an image-receiving sheet, etc. by application of heat and pressure, the support (temporary support) of the transfer sheet is readily peeled from the peeling layer. This is because the peeling layer does not substantially absorb moisture. In other words, the transfer operation can be carried out without being affected by high humidity. Furthermore, the development step for forming the separation image on the photosensitive transfer sheet leaves little stain on the peeling layer due to colorants such as dyes and pigments, to thereby markedly prevent colorant fog.
However, preparation of gang proofs (proofs made according to a multiple face setting method) requires cutting the exposed and developed transfer sheets with scissors before carrying out the transfer steps for the second and subsequent colors for positioning in accordance with the corresponding originals. The transfer sheets having the above-described peeling layer tend to generate a powder upon cutting, and this cutting powder occasionally adheres to the transfer sheets. Because the cutting powder is taken like dusts or foreign substances in a color proof thus prepared, the transfer sheet significantly degrades the quality of the color proof.
Furthermore, when the photosensitive transfer sheets are large, the exposed and developed sheets tend to generate kink marks on handling. This causes film floating (a phenomenon where an image formed on a peeling layer is peeled together with the peeling layer from a support) in portions where the kink marks are generated and image disruption (an image formed on a peeling layer is broken off together with the peeling layer by cracking).
Furthermore, the above-mentioned cutting powder, film floating and image disruption become pronounced at low humidity, and these problems arise, for example, during the winter when humidity decreases. JP-A-3-126037 describes a peeling layer made of an alcohol-soluble polyamide containing a basic compound and an alkali-soluble organic polymer. However, when the basic compound is added in an amount needed to reduce the above-described cutting powder, film floating, image disruption, and the like, the transfer properties deteriorate.